The improvement in the durable temperature of super-heat-resisting alloy which should be used at a high temperature has been recognized as a pressing need for energy-saving. The increase in the amounts of elements to be added in order to satisfy this requirement has a disadvantage that it lowers the workability of super-heat-resisting alloy material.
The superplastic forming method is a process which is capable of solving this problem. It is a process for obtaining a work having a complex configuration with an extremely large amount of deformation by processing the material under conditions that it can show the superplasticity.
Superplastic forming has following characteristics: (1) A material can be deformed at a low stress level. Therefore, vacuum forming and gas pressure forming can be used. (2) The deformability is so large as to allow the material to take a complex configuration. Then, the machining cost can be saved. (3) Since the work does not have such a residual stress as generated upon cold working, the corrosion resistance is improved and the precision level of the size is maintained to be stable. (4) The surface of a work is in a good state even after working. Therefore, the superplastic forming method has an advantage that it is appropriate for the forming of such an alloy as is difficult to work with ordinary forming processes.
Superplastic deformation which has these characteristics is broken down into two types; one of which makes use of micrograin superplasticity and the other makes use of transformation superplasticity. The superplastic deformation process to be used in this invention is the process which makes use of the former type of superplasticity, according to which it becomes necessary to produce a material for superplastic forging having grain size level below several micrometers.
The powder metallurgy, utilizing the atomization process or others which have been developed recently, makes it possible to produce such a material for superplastic forming mentioned above. The present invention relates to a production of a super-heat-resisting alloy material, specially to a material appropriate to the superplastic deformation which makes use of powder metallurgy.
Priorly, a material for superplastic deformation has been produced by either powder extrusion, according to which the alloy powder is extruded at a temperature just below the recrystalization temperature and is allowed to recrystalize by the heat generated upon the extrusion so as to have a micrograin structure of grain size of 10 .mu.m or less, or hot isostatic pressing (HIP) according to which the alloy powder is filled in a capsule and then is consolidated under the conditions of high pressure and high temperature.
However, the powder extrusion process has a disadvantage that the production of a large material requires a large-scale and very costly extrusion machine. On the other hand, according to the HIP process, there are invited such disadvantages that the absorbed gas contamination on the surface of the powder is confined in the material due to the air-tight seal of the capsule so that the trapped gas affects the characteristics of deformation on superplastic forging and deteriorates the deformability, and an that the air-tight sealing upon filling the powder into the capsule is difficult. No leaks should be allowed, and so we should pay attention to every seal position of the capsule, especially to welded positions. Even slight leak allows the high pressure gas to enter into the capsule. Then, the gas is confined in invisible voids when the powder is consolidated into a completely densified state and spreads in the material during the heat treatment at a high temperature to affect the mechanical properties of a product undesirably.